This invention relates to optical systems and more particularly to an optical system for insertion into projection apparatus to permit viewing of an image projected on an image plane. The optical system of the invention is particularly adapted, among other possible uses, for use as an insertion in the optical train used in microprojection apparatus to provide access to the image of a mask on a light sensitive wafer in order to closely align the image with the pattern on the wafer. It will be appreciated that such insertion of an optical system must not affect the quality, size, or position of the image. A good example of an optical train used in microprojection apparatus is shown and described in the Offner U.S. Pat. No. 3,748,015, issued July 24, 1973.